Stitch-forming machine with a transducer and a control device

ABSTRACT

A stitch forming machine is provided including a transducer for determining the tension present in a thread, wherein the thread tension assumes a higher value during stitch formation and the transducer provides a signal representing the tension level. The control is provided for evaluating the signal corresponding to the tension level. The control includes a comparator device for comparing a peak of the signal representing the tension level, which peak can be used to detect a malfunction, with a limit signal, corresponding to a limit tension. The comparator sends a signal to a switching device when a signal peak drops below the limit signal. The switching device may be connected to a shut-off device of the drive motor of the machine as well as one or more display elements. In this way, the machine may be stopped and the display element associated with a limit tension, below which the tension dropped by a switching device. The comparator may, according to another aspect of the invention, only provide a signal for energizing a switch depending upon a stitch formation phase associated with a signal peak which drops below a limit signal.

FIELD OF THE INVENTION

The present invention pertains generally to stitch-forming machines andmore particularly to a thread monitor for indicating missed stitches.

BACKGROUND OF THE INVENTION

A thread monitor, known from U.S. Pat. No. 4,170,951, is arranged on asewing machine in the path of the needle thread and is provided with atransducer with a spring clip, to which a wire strain gauge element,hereinafter called a WSG element, is fastened. The WSG element producesan electrical voltage that is proportional to its mechanical deformationcaused by the deflecting movement of the spring clip. The electricalvoltage is fed into an evaluating electronic unit following thetransducer.

In the course of a stitch, a first, lower tension level is producedduring the expansion of the needle thread loop, and a second, highertension level is generated during the knotting. To detect missedstitches, the tension levels are monitored in measuring windows, whosepositioning and size are predetermined by two signal generators thatmonitor the position of the arm shaft.

A plurality of actual values are determined from the lower tensionlevel, and compared with a threshold value, whose value depends on themaximum of the higher tension level generated during the precedingstitch. If all actual values are below this threshold value, a warningsignal is sent by the evaluating electronic unit to indicate a missedstitch.

Since the maximum of the higher tension level may differ from thecorresponding value of the preceding stitch during each stitch, and thethreshold value depends on this maximum, the threshold value is to bedetermined anew for each stitch. Such a signal evaluation is problematicespecially at high sewing speeds and requires the use of an expensiveevaluating electronic unit.

Due to its dependence on the preceding higher tension level, thethreshold value cannot be formed during the first stitch performed withordinary tension after start-up of the sewing machine, which causes adelay in the monitoring process. To prevent this disadvantage,additional control elements not disclosed in U.S. Pat. No. 4,170,951 arenecessary. This increases the circuit complexity of the evaluatingelectronic unit, which is suitable exclusively for indicating missedstitches caused by lack of expansion of the needle thread loop.

SUMMARY AND OBJECTS OF THE INVENTION

It is a primary object of the present invention to design a controldevice of a stitch-forming machine equipped with a transducer, so thatthe control device is able to evaluate the measured values sent by thetransducer, at low circuit complexity, beginning from the first stitchperformed with ordinary tension to detect a majority of different missedstitches as well as thread disturbances on the thread being monitoredand on the threads to be connected to this by stitch formation.

According to the invention, a stitch forming machine is providedincluding a transducer for determining the tension present in a thread,wherein the thread tension assumes a higher value during stitchformation and the transducer provides a signal representing the tensionlevel. Control means are provided for evaluating the signalcorresponding to the tension level. The control means includes acomparator device for comparing a peak of the signal representing thetension level, which peak can be used to detect a malfunction, with alimit signal, corresponding to a limit tension. The comparator sends asignal to a switching device when a signal peak drops below the limitsignal. The switching device may be connected to a shut-off device ofthe drive motor of the machine as well as one or more display elements.In this way, the machine may be stopped and the display elementassociated with a limit tension, below which the tension dropped by aswitching device.

According to another aspect of the invention, a stitch forming a machineis provided included a transducer for determining the tension in athread and for outputting a signal representing the tension of thethread. Control means are provided for evaluating the signalrepresenting the tension of the thread, the tension of the threadassuming higher values while stitches are formed. Control means areprovided for evaluating the thread tension based on the stitch formationstage. The control means includes a comparator device for comparing apeak in the transducer signal corresponding to a peak in the tensionlevel, the peak being used to detect a malfunction, with apredeterminable, common limit signal representing a common limittension. The comparator provides a signal for energizing a switchdepending upon a stitch formation phase associated with a signal peakwhich drops below a limit signal. The switching device is connected to ashut-off device of a drive motor of the machine or a plurality ofdisplay elements so that the machine can be stopped and the displayelement associated with the phase of stitch formation can be switched bythe switching device.

The control device according to the present invention makes it possibleto detect a plurality of different malfunctions on the thread, such asdifferent types of missed stitches or the break of the needle threadand--in the case of double lockstitch machines as well as multi-threadchain stitch machines--the break of the hook or looper thread by meansof a single transducer, because such a malfunction is positivelydemonstrable by the change in the value of the tension peak associatedwith this.

Monitoring the tension peaks by a comparator device, according to theinvention, is advantageous if at least one of the tension levels has aplurality of tension peaks. Since not every of these voltages peaks isusually suitable for detecting a malfunction, only those peaks fromwhich a malfunction is recognizable are monitored. This makes itpossible to reduce the monitoring time to a minimum.

Since a plurality of different malfunctions can be recognized bymonitoring the tension peaks, it is advantageous to stop the PG,7machine in the case of a malfunction and to indicate the malfunction bya separate display element associated with this. The display element,which can be switched via the switching device, may be designed as anoptical or acoustic warning device.

By presetting the limit tension associated with the actual tension peakaccording to one aspect of the invention or the limit tension that isuniform for all tension peaks according to another aspect of theinvention, it is ensured that the tension peaks can be monitored evenduring the first stitch performed with ordinary tension, because thecorresponding limit voltage can immediately be associated with eachtension peak by the comparator device.

By presetting a limit tension adjusted to the respective tension peak bythe comparator device, the tension value below which a malfunction isrecognizable can be individually adjusted to the maximum of the tensionpeak, so that a malfunction can be indicated as quickly as possibleafter it appears, but variations in the values of the voltage peaks thatare caused by the sewing technique bring about no switching process.

In the design of the comparator device according to another aspect ofthe invention, a common limit tension is set for all tension peaksregardless of their values in order to simplify the circuit. The phaseof stitch formation of the machine, which is sent as a signal to thecomparator device, is needed as a second unit of information. Thereduction of the voltage peak below the limit tension is used to detecta malfunction here, while the nature of the malfunction can bedetermined from the phase of stitch formation associated with thetension peak.

A particularly advantageous application of the control device accordingto the present invention is disclosed in which different missed stitchescaused by a pick-up or stitch-down error, as well as a break or end ofthe needle thread and hook or looper thread can be recognized bymonitoring the tension peaks that indicate the correspondinginformation.

Measurement experiments have shown that such parameters as the speed ofsewing, stitch length, and the thread properties cause onlyinsignificant changes in the maximum of the voltage peaks, whereas thesetting of the tensioning device substantially affects it. To rule outdisadvantages during thread monitoring in the case of a changed settingof the tensioning device, the limit tension and consequently also theresponse threshold of the comparator device are adjusted to the threadvoltage set by the adjusting device.

Due to the measure of providing a spring member that can be deflected bythe tensioned thread with a sensor device responsive to the proportioneddeflection wherein the spring element tapers towards its free endbeginning from its point of clamping, the spring element has the lowestpossible weight at a predetermined bending strength. As a result, theeffect of the natural oscillations of the spring element on the valuesof the thread tension sent to the control device will be negligible evenat high sewing speeds.

The measure of fastening the transducer on the machine via a dampingelement, reduces the oscillations transmitted from the machine to thetransducer to a negligible level, so that the values of the threadtension are not distorted by these oscillations.

The measure of providing the transducer arranged immediately downstreamof a tensioning device, with respect to a direction of pull, leads tothe variations in tension brought about by stitch formation beingreduced to a minimum, which might cause distortion of the thread tensiontransmitted to the control device.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 in a side partially sectional view of a sewing machine with athread monitor equipped with a transducer according to the invention;

FIG. 2 is a partially sectional view showing the transducer according toFIG. 1 on a larger scale;

FIG. 3 is a circuit diagram showing a simplified control deviceaccording to the invention;

FIGS. 4a through 4g are diagrams representing the following processesrelative to one stitch:

FIG. 4a: thread voltage (U_(F)) without malfunction,

FIG. 4b: thread voltage (U_(F)) during a pick-up error or disturbance onthe needle thread,

FIG. 4c: thread voltage (U_(F)) during a stitch-down error or adisturbance on the hook or looper thread,

FIG. 4d: comparator voltage (U_(K)) without malfunction,

FIG. 4e: comparator voltage (U_(K)) during a malfunction according toFIG. 4b,

FIG. 4f: comparator voltage (U_(K)) during a malfunction according toFIG. 4c,

FIG. 4g: impulses (I) of a position transmitter;

FIG. 5 is a partially sectional view showing a second embodiment of thetransducer on a larger scale;

FIG. 6 is a circuit diagram showing a simplified second embodiment ofthe control device; and

FIG. 7 is a diagram showing the thread voltage (U_(F)) withoutmalfunction according to the second embodiment of the control device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A tensioning device 2 for the needle thread coming from a thread reserve(not shown) is arranged on the stand 1 of the double-thread chainstitchsewing machine shown in FIG. 1. A transducer 3, which is fastened to thesewing machine via a damping element 4 (FIG. 2) made of, e.g., rubber,is provided behind the tensioning device 2 in the direction of threadpull. The transducer 3 has a bending bar 5, whose width is reducedtoward the free end beginning from the clamping point. At its free end,the bending bar 5 is designed on the underside with a needlethread-receiving eye 6 on it.

Wire strain gauge elements, hereinafter called WSG elements 7, areprovided to receive the tension of the needle thread. A first WSGelement 7 is fastened on the top side and a second WSG element 8 on theunderside of the bending bar 5 close to the point of clamping of thebending bar.

The WSG elements 7 and 8 are connected to a power source (FIG. 3) andare connected to form a half bridge 9 which is connected to an amplifier10. The output of the amplifier 10 is connected to a voltmeter 11 with adisplay unit 12 and to a comparator 13 with an adjusting device 14serving to set its switching threshold.

The output of the comparator 13 is connected to one input of ANDelements 15 and 16 each, whose second input is connected to a positiontransmitter 18 that counts the revolutions of the main shaft 17. Thisposition transmitter 18 has a photodiode 19, which is connected to thepositive pole of a stabilized power source, is grounded via a resistor20, and has a photodetector 21, which is designed as a phototransistor,is also connected to the positive pole, and is grounded via a resistor22. The position transmitter 18 is also provided with a photodiode 23,which is connected to the positive pole of the power source, is groundedvia a resistor 24, as well as with a photodetector 25, which is alsoconnected to the positive pole, is designed as a phototransistor, and isgrounded via a resistor 26. A disk 27, which is arranged nonrotatably onthe main shaft 17, is provided between the photodiodes 19 and 23 and thephotodetector 21 and 25; the disk 27 has--in the light path between thephotodiode 19 and the photodetector 21--a first opening 28 and, onanother radius, in the light path between the photodiode 23 and thephotodetector 25, a second opening 29 for passage of the light beams.During each passage through the opening 28, an impulse is sent to theAND element 15, and during each passage through the opening 29, animpulse is sent to the AND element 16, and the AND element 16 isenergized for a period corresponding to rotation of the disk 27 through180° after the AND element 15.

The output of the AND element 15 is connected to the setting input S ofa flip-flop memory 30, and that of the AND element 16 is connected tothe setting input S of a flip-flop memory 31. The AND elements 15 and 16form, together with the memories 30 and 31, a switching circuit 32.

The output Q of the memory 30 is connected to a display element 33,which is grounded via a resistor 34, while a display element 35, whichis grounded via a resistor 36, is connected to the output Q of thememory 31. In addition, a switch 37, which is connected to a shut-offdevice 38 of a drive motor 39, is connected to the outputs Q of thememories 30 and 31. The drive motor 39 drives said main shaft 17 via atoothed belt.

The elements 10 through 37 form a control device 40, which is providedfor evaluating the thread voltage (U_(F)) measured by the transducer 3.

Behind the transducer 3 in the direction of thread pull (FIG. 1), afirst thread guide element 42 is fastened on the sewing machine, and asecond thread guide element 44 is fastened on the head 43. The needlethread is fed by the thread guide element 44 to the needle 48 via athread lever 45 and further thread guide elements (not shown), as wellas an eye 47 provided on a needle bar 46. A chain stitch looper 51 isarranged beneath the needle plate 50 accommodated in the base plate 49.The looper thread is fed to the looper 52 via a tensioning device 52fastened on the stand 1 as well as thread guide elements (not shown).

The elements 45, 46, 48, and 51 will hereinafter be calledstitch-forming elements 53.

The device operates as follows:

During sewing, the needle thread and the looper thread are pulled offfrom the thread reserve, while the tension of the threads variesdepending on the movement of the stitch-forming elements 53. Since theneedle thread and the looper thread are to be linked with one another bythe stitch formation in terms of tension, one transducer 3 in the pathof the needle thread is sufficient to determine the changes in thethread voltage (U_(F)) formed from the voltages of all thread.

FIG. 4a shows the changes in the thread tension (U_(F)) duringtrouble-free stitch formation during one stitch.

The first tension level (U_(p1)) exceeding the normal tension (U_(N)) isformed when the loop of the needle thread is caught and expanded by thelooper 51 after the needle 48 has passed through a material being sewn.The first tension level (U_(p1)) reaches its tension peak (U₁) at thetime (t₁).

The second tension level (U_(p2)) is formed when the thread lever 45performs an upward movement to tighten the loop formed by the needlethread and the looper thread. The tension level (U_(p2)) has two tensionpeaks (U₂,1 and U₂,2) at the times (t₂ and t₃), and the value of thefirst tension peak (U₂,1) exceeds that of the second tension peak(U₂,2).

When the looper 51 misses the needle thread loop, a pick-up erroroccurs. In the case of such an error or break of the needle threadbehind the tensioning device 2 in the direction of thread pull, thethread voltage (U_(F)) changes according to FIG. 4b. The first tensionlevel (U_(p1)) assumes the value of the normal tension (U_(N)) or evendrops below this value, while the second tension level (U_(p2)) isformed only with one tension peak (U₂).

Should said needle 48 miss the loop formed by the looper thread afterpassing through the material being sewn, a stitch-down error occurs.Like the break of the looper thread behind said tensioning device 52 inthe direction of thread pull, this is indicated by a change in thethread tension (U_(F)) according to FIG. 4c). Just like the firsttension peak (U₂,1) of the second tension level (U_(p2)), the firsttension level (U_(p1)) remains nearly unchanged, whereas the value ofthe second tension peak (U₂,2) is greatly reduced.

The transducer 3 (FIG. 1) is arranged between the tensioning device 2and the thread guide element 42 so that the needle thread is deflectedwhile passing through the eye 6. As a result, a force perpendicular tothe direction of extension of the bending bar 5, by which the bendingbar is deflected in the downward direction, is generated. As aconsequence of this deflection, which is proportional to the threadtension (U_(F)), the WSG element 7 is tensioned on the top side of thebending bar 5, and the WSG element 8 on its underside is compressed, sothat the electrical resistance of both WSG elements 7 and 8 will change.As a result, a differential tension (U_(D)) is formed, which isproportional to the deflection of said bending bar 5 and whose changesduring one stitch correspond to those of the thread tension (U_(F)).

After amplification by the amplifier 10 (FIG. 3), the differentialvoltage (U_(D)) is sent to the voltmeter 11, which displays its value,as well as to the comparator 13. Depending on the setting of thetensioning device 2, the switching threshold of the comparator 13 can beadjusted by means of the adjusting device 14, so that its sensitivity isadjusted to the tension of the needle thread. The switching threshold isselected so that one of the tension peaks (U₁, U₂,2) will drop below itonly when malfunction, such as a missed step or thread break, hasoccurred. The tension corresponding to the switching threshold willhereinafter be called the limit tension corresponding to a limit voltage(U_(G)), which is shown in FIGS. 4a through 4c.

The comparator 13 is turned on as long as the differential voltage(U_(D)) present at its input is lower than the limit voltage (U_(G)),and is turned off as soon as the differential voltage (U_(D)) assumes orexceeds the value of the limit voltage (U_(G)) FIG. 4d shows the changesin the output voltage (U_(K)) of said comparator (13) as a function ofthe differential voltage (U_(D)) according to FIG. 4a, while the changesin the output voltage (U_(K)) according to FIG. 4e are associated withthose of the differential voltage (U_(D)) according to FIG. 4b, and thechanges in the output voltage (U_(K)) according to FIG. 4f areassociated with those of the differential voltage (U_(D)) according toFIG. 4c.

As long as no malfunction has occurred, the comparator output voltage(U_(K)) is present at the input of the AND elements 15 and 16 when noneof the impulses (I₁ or I₂) shown in FIG. 4g, which are sent by theposition transmitter 18, arrives. As a result, no signal is able toleave the AND elements 15 and 16.

In the case of the malfunction according to FIG. 4b, the impulse (I₁) ofthe position transmitter 18 arrives at time (t₁) from the photodetector21 to one input of the AND element 15 when the comparator voltage output(U_(K)) is present at its other input. A signal is then sent from theoutput of the AND element 15 to the setting input S of the memory 30.The signal causes the memory 30 to turn on, via its output Q, thedisplay element 33, which will display a pick-up error or the break ofthe needle thread. With the switch 37 closed the output Q of the memory30 activates at the same time the shutoff device 38, which, depending onthe design, turns off the drive motor 39 immediately, or prevents itfrom restarting after the next stoppage.

After a resetting switch (not shown) has been activated, an electricalimpulse is sent in a suitable manner to the resetting input (R) of thememory 30, so that this will turn off the display element 33 and releasethe drive motor 39.

In the case of a malfunction according to FIG. 4c, said photodetector 25of the position transmitter 18 sends an impulse (I₂) at time (t₃) to oneinput of the AND element 16, while the comparator voltage (U_(K)) ispresent at its other input. As a result, the AND element 16 is connectedthrough, and sends from its output a signal to the setting input S ofthe memory 31, so that this will turn on, via its output Q, the displayelement 35, which will display a stitch-down error or a break of thelooper thread. With the switch 37 closed, the output Q of the memory 31at the same time activates, like that of the memory 30, the shutoffdevice 38 of the drive motor 39. The display element 35 is turned off byan electrical signal sent to the resetting input R of the memory 31, andthe drive motor 39 is released.

FIG. 5 shows a second embodiment of the transducer 3. A permanent magnet54 is fastened on the top side of the bending bar 5 at its free end. AHall sensor 56 is fastened at the free end of a bracket 55, facing thepermanent magnet 54.

During the downward deflection of the bending bar 5 under the action ofthe needle thread, the distance between the permanent magnet 54 and theHall sensor 56 is increased, as a result of which the magnetic fluxdensity and thus also the Hall voltage of the Hall sensor 56 will bereduced corresponding to the deflection of the bending bar 5. The Hallvoltage is sent to and evaluated in the control device 40.

FIG. 6 shows a second embodiment of the control device 40. The output ofthe amplifier 10 is connected to the voltmeter 11 and, via an A/Dconverter 57, to one input E1 of a microprocessor 58. An input device 59is connected to a second input E2 of the microprocessor 58.

The microprocessor 58 has outputs A1 and A2, of which the output A1 isconnected to the setting input S of a flip-flop memory 60, and theoutput A2 is connected to the setting input S of a flip-flop memory 61.The memories 60 and 61 form a switching device 62.

The output Q of the memory 60 is connected to the display element 33,and that of the memory 61 is connected to the display element 35. Bothoutputs Q are also connected to the shutoff device 38 of the drive motor39 via the switch 37.

The second embodiment of the control device 40 operates as follows:

After amplification in the amplifier 10, the differential voltage(U_(D)) (FIG. 7) is sent to the A/D converter 57. A digital voltage,which is proportional to the differential voltage (U_(D)) present on theinput of the A/D converter 57, is present at the output of the A/Dconverter 57.

The digital voltage received at the input E1 is evaluated by themicroprocessor 58 only at the time intervals in which the tension levelswith proportional voltage (U_(p1) and U_(p2)) are formed.

The microprocessor 58 determines the value from all the digital voltagesassociated with the first tension level (U_(p1)), and forms the maximum(U_(M1)) from these values. The maximum (U_(M1)) is compared with afirst threshold value, which is associated with a first limit tensioncorresponding to a first limit voltage (U_(G1)) (FIG. 7). The limitvoltage (U_(G1)) is to be preselected on the input device 59 dependingon the setting of the tensioning device 2, and is sent to themicroprocessor 58 via its input E2.

As long as the maximum (U_(M1)) corresponds to or exceeds the firstthreshold value, no signal is sent by the microprocessor 58. However,when the maximum (U_(M1)) drops below the first threshold value as aconsequence of a pick-up error or a disturbance on the needle thread,the microprocessor 58 sends an impulse from the output A1 to the memory60, as a result of which the memory 60 is switched over, and activatessaid display element 33 via its output Q and, with the switch 37 closed,it activates the shutoff device 38 of the drive motor 39.

The maximum (U_(M2)) is formed from the values of the digital voltagesassociated with the second tension peak corresponding to voltage peak(U₂,2) of the tension level corresponding to voltage level (U_(p2)) andcompared with a second threshold value, which is associated with asecond limit tension corresponding to a limit voltage (U_(G2)) (FIG. 7).Like the limit voltage (U_(G1)), this is to be preselected on the inputdevice 59 depending on the setting of the tensioning device 2.

When the maximum (U_(M2)) corresponds to or exceeds the second thresholdvalue, no signal is sent by the microprocessor 58. However, when themaximum (U_(M2)) drops below the second threshold value as a consequenceof a stitch-down error or a disturbance on the looper thread, themicroprocessor 58 sends from its output A2 an impulse to the memory 61.As a result, this is switched over, and controls, via its output Q, thedisplay element 35 and the shutoff device 38 of the drive motor 39.

The memories 60 and 61 can be switched over to their starting positionby an electrical signal to the resetting input (R).

By presetting different limit voltages (U_(G1), U_(G2)) for thedifferent maxima (U_(M1) and U_(M2)), the respective threshold value canbe optimally adjusted to the corresponding maximum.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A stitch-forming machine, comprising:a transducerfor determining tension present in a thread, the tension assuming ahigher value during stitch formation, the transducer generating atension signal representing a thread tension level; and control meansfor evaluating the thread tension level, said control means including acomparator for comparing peaks in said tension signal, wherein a peakcan be used to detect a malfunction, with a limit signal representing atension limit, and for generating a switching signal when a tensionsignal peak drops below the limit signal and including a switchingdevice connected to a shutoff device of a drive motor of the stitchforming machine and to a display element, said switching signal stoppingthe machine and activating the display element.
 2. A stitch-formingmachine, comprising:a transducer for determining the tension in a threadand generating a signal representing the thread tension; and controlmeans for evaluating the thread tension with respect to a phase ofstitch formation, the control means including a comparator device forcomparing voltage peaks occurring during said phase, corresponding tothread tension peaks, of each tension signal with a predeterminablecommon limit signal, corresponding to a common limit tension and forgenerating a comparator output switching signal when a tension signalpeak drops below the limit signal and including a switching deviceconnected to a shutoff device of a drive motor of the machine andconnected to a display element, said control means allowing saidcomparator output switching signal to energize said switching devicedepending upon a stitch formation phase, for stopping the machine andactivating the display element.
 3. A stitch-forming machine according toeither claim 1 or claim 2, wherein said stitch forming machine forms achain stitch seam having a thread tension with a first tension levelduring an expansion of a needle thread loop and having a second tensionlevel during a knotting of the needle thread loop, said switching devicebeing energized by said comparator switching signal when said tensionsignal drops below said limit signal for switching and a pick-up errordisplay element for detecting a pick-up error or a disturbance of theneedle thread and when the tension signal of the second tension level,which has two tension peaks, drops below an additional limit signal, asecond display element is activated for detecting a stitch-down error,or, in the case of a multi-thread chain stitch machine, a disturbance ofa looper thread.
 4. A stitch-forming machine according to either claim 1or claim 2, wherein said comparator device includes an adjusting devicefor providing a predetermined limit signal as a function of a setting ofa tensioning device associated with the thread being monitored.
 5. Astitch-forming machine according to either claim 1 or claim 2, whereinsaid transducer comprises a spring member connected to the tensionedthread, said spring member being deflected by said tensioned thread anda sensor device generating said tension signal in proportion to adeflection of said spring member, said spring member being formed as abending bar which tapers toward a free end beginning from a clampingend.
 6. A stitch-forming machine according to either claim 1 or claim 2,wherein said transducer is fastened on the machine via a dampingelement.
 7. A stitch-forming machine according to either claim 1 orclaim 2, wherein said transducer is arranged immediately downstream ofsaid tensioning device.
 8. A stitch-forming machine, comprising:atransducer for determining the tension in a thread and generating asignal representing the thread tension; and control means for evaluatingthe thread tension with respect to a phase of stitch formation, thecontrol means including a comparator device for comparing voltage peaks,corresponding to thread tension peaks, of each tension signal with apredeterminable common limit signal, corresponding to a common limittension and for generating a comparator output signal when a tensionsignal peak drops below the limit signal and including a switchingdevice connected to a shutoff device of a drive motor of the machine andconnected to a display element, said control means allowing saidcomparator output signal to energize said switching device dependingupon a stitch formation phase, for stopping the machine and activatingthe display element, wherein said stitch forming machine forms a chainstitch seam having a thread tension with a first tension level during anexpansion of a needle thread loop and having a second tension levelduring a knotting of the needle thread loop, said switching device beingenergized by said comparator signal when said tension signal drops belowsaid limit signal for switching and a pick-up error display element fordetecting a pick-up error or a disturbance of the needle thread and whenthe tension signal of the second tension level, which has two tensionpeaks, drops below an additional limit signal, a second display elementis activated for detecting a stitch-down error, or, in the case of amulti-thread chain stitch machine, a disturbance of a looper thread. 9.A stitch forming machine comprising:a transducer for determining tensionpresent in a thread, the tension assuming a higher value during stitchformation, the transducer generating a tension signal representing athread tension level; needle position means for generating a signalrepresenting one or more predetermined positions of the needle duringstitch formation, said one or more predetermined positions normallycorresponding to said higher value of the tension resulting in peaks insaid tension signal; and control means for evaluating the thread tensionlevel, said control means including a comparator for comparing saidthread tension level with a thread tension peak limit signalrepresenting a thread tension peak limit, upon receiving said threadneedle position signal, said thread tension peak limit having a valuehigher than a normal thread tension, occurring during a period of nostitch formation, and for generating a switching signal when said threadtension level signal, during said needle position signal, drops belowsaid tension peak limit signal.
 10. A stitch-forming machine accordingto claim 9, wherein said stitch forming machine forms a chain stitchseam having a thread tension with a first tension level during anexpansion of a needle thread loop and having a second tension levelduring a knotting of the needle thread loop, said switching device beingenergized by said switching signal when said tension signal drops belowsaid limit signal for switching and a pick-up error display element fordetecting a pick-up error or a disturbance of the needle thread and whenthe tension signal of the second tension level, which has two tensionpeaks, drops below an additional limit signal, a second display elementis activated for detecting a stitch-down error, or, in the case of amulti-thread chain stitch machine, a disturbance of a looper thread. 11.A stitch-forming machine according to claim 9, wherein said comparatordevice includes an adjusting device for providing a predetermined limitsignal as a function of a setting of a tensioning device associated withthe thread being monitored.
 12. A stitch-forming machine according toclaim 9, wherein said transducer comprises a spring member connected tothe tensioned thread, said spring member being deflected by saidtensioned thread and a sensor device generating said tension signal inproportion to a deflection of said spring member, said spring memberbeing formed as a bending bar which tapers toward a free end beginningfrom a clamping end.
 13. A stitch-forming machine according to claim 9,wherein said transducer is fastened on the machine via a dampingelement.
 14. A stitch-forming machine according to claim 9, wherein saidtransducer is arranged immediately downstream of said tensioning device.